The molybdenum-containing enzyme sulfite oxidase catalyzes the conversion of sulfite to sulfate, the terminal step in the oxidative degradation of cysteine and methionine. Deficiency of this enzyme in humans usually leads to major neurological abnormalities and early death. Chicken liver sulfite oxidase has 68% sequence identity with the human enzyme, and the structure of the chicken liver enzyme has been solved from data collected at SSRL to 1.9 E resolution. The structure reveals that the protein is dimeric, and the Mo atom pentacoordinated by three sulfur ligands, an oxo group and one water/hydroxo ligand. A sulfate molecule adjacent to the Mo identifies the substrate binding pocket. The structure enables four variants associated with sulfite oxidase deficiency in humans to be related to structure. Two are near the sulfate binding site; two occur within the domain mediating dimerization. The distance between the Mo and Fe of the heme of 32 E is surprisingly long considering the rates of intramolecular electron transfer in the protein.